This invention relates to water retention additives for aqueous construction systems. Water retention additives are chemical materials which, when added in relatively small amounts to a system, improve the system's workability, lubricity, smoothness and crack resistance as the result of delayed drying and other qualities. Such additives are often referred to as water retention aids.
Aqueous construction compositions where such products have proved useful include joint compounds, pastes, construction grouts, stuccos and other water-miscible construction systems. It is generally understood, for example, that water retention control agents are one of the most essential components in joint compounds.
Commercial additives for joint compounds available today are generally cellulosic-based products. Such water retention aids are expensive and, at the same time, have often failed to provide the necessary water retention properties to achieve smooth application of the joint compound without premature drying, and proper viscosity, as is demanded by modern construction contractors. It is highly desired that such systems be flowable during prolonged application, but that they not sag or separate after they have been applied. It is further desired that, while being applied, the compound remain on the substrate and not slide. It is still further desired that such systems be stable over a range of temperatures (and for a sufficient period of time), so that their properties do not change upon storage, including storage at non-ambient temperatures.
Polysaccarides, such as methyl cellulose and hydroxyethyl cellulose, have dominated the joint compound market almost since the elimination of the use of asbestos. Methyl cellulose ("MC") is a common, nonionic, water-soluble material for such water-based systems. However, MC suffers from several disadvantages not the least of which is its very high cost. In some formulations, MC is responsible for the poor crack resistance obtained. MC is also subject to bacterial and enzymatic-induced degradation ("biodegradation") resulting in loss of viscosity, thus requiring the incorporation of anti-biodegradation preservatives when construction projects are undertaken. In addition, because these types of additives are well known to form lumps in water and are not readily dispersed, the proper addition of MC water retention additives require prehydration in water before addition, resulting in long mixing, and dilution times, and extra production steps. The resulting delays increase processing cost.
For these reasons and others, a search has long continued for a water retention product to replace cellulosics, which is not biodegradable and is without toxic by-products. Also searched for has been a product with a lower overall manufacturing cost and equivalent performance with respect to the water-retention, delayed-drying properties of joint compound compositions and similar compounds into which the additive is used.
Construction Compositions
An important use of water-retention additives is in joint compounds for the construction industry. Starting in the 1940s, the use of wallboard in the United States has largely displaced the use of plaster in interior walls in the construction of buildings. Wallboard is generally installed in panels which are nailed, glued or screwed to the studding of the wall and fitted together until the entire section of wall is covered. The joints where the sections of wall board come together are covered with tape and then the taped joint (and all blemishes, including those caused by nails or other fasteners) are covered with a joint compound (also sometimes called a joint cement) which, upon hardening, can be sanded smooth so that the taped joint cannot be detected when the wall is covered with paint or, in more costly projects, with wallpaper. The process often includes the application of a first coating and a layer of non-adhesive tape which, when dried, is coated by a second joint compound layer. This layer may be sanded and a third layer may be applied and conventionally finished. A variety of joint compounds are used according to the various applications such as dry grade, taping grade, topping grade, all-purpose Grade, ready mix grade and the like. Joint compounds typically are prepared as water-based formulations, the composition of which varies because of geography and climatic conditions.
While joint materials may contain a variety of materials, water retention additives, fillers, binders and thickeners typically must be present. For example, in U.S. Pat. Reissue No. 29,753, cellulosic materials and clay minerals are included in a joint compound formulation wherein the water retention additives discussed include hydroxypropylmethyl cellulose, hydroxyethyl cellulose, alginates and sodium carboxymethyl cellulose. Attapulgite clay is listed as a non-leveling agent.
At first, such joint compounds contained a binder, limestone, mica, and asbestos as their principal ingredients, and were mixed with water, to form a viscous, gel-like, workable substance. Asbestos was included in these compounds for several purposes, with its primary functions being to impart pseudoplasticity and to prevent sag of the compound when applied in a thick layer on a partical surface.
Evidence that certain types of asbestos appear to have carcinogenic characteristics has led to its present ban from almost all of its traditional applications. Both prior to and when the ban became effective, efforts were made to find substitutes in joint compounds for asbestos; many materials were evaluated as asbestos replacements. Today, the overwhelming product of choice in the joint cement industry to replace asbestos is attapulgite clay, although bentonite clay is also frequently used. However, such clays are added as inexpensive fillers, and not to prevent sag in the joint compound or to provide water retention properties.
Attapulgite clay is acicular and has an extremely high surface area in relationship to its weight. Hence, water-soluble cellulosic polymers used as water retention aids in joint cements are attracted to the surface of the clay particles to some degree. The extent or degree of adsorption of the water-soluble polymer on the surface of the attapulgite clay is related to the type of the cellulose and its hydrogen bonding capability, and often results in diminished effectiveness.
Since the elimination of asbestos, methyl cellulose, methylhydroxypropyl cellulose, hydroxyethylcellulose (HEC) and a variety of other modified cellulosics have been used as water retention additives in joint compounds. For example, U.S. Pat. Nos. 4,558,079 and 3,891,582 describe joint compounds using mixtures of these products to impart pseudoplasticity to the compound described.
Alkali-Swellable Polymers
A number of alkali-swellable polymers have been used in the past as rheological additives or thickeners to provide thickening and viscosity modification to aqueous systems, most commonly paints and coatings.
One of the first such polymers is described in Rheox U.S. Pat. No. 4,226,754, issued in 1980 which, in a preferred embodiment, discloses a polymer made by the reaction of (a) an ester of methacrylic acid, (b) methacrylic acid and (c) a vinyl ester of a saturated aliphatic carboxylic acid. This type of thickener has often been referred to as an alkali-swellable or alkali soluble latex copolymeric thickener, as it contains carboxylic acid groups in sufficient quantity to render the polymer soluble in water following neutralization with a water-soluble base. Rheox, Inc., assignee of the instant invention, sells a product, designated "RHEOLATE 1 ", as a rheological additive which is an embodiment of the invention of U.S. Pat. No. 4,226,754.
U.S. Pat. No. 4,138,381 shows a reaction product of an unsaturated carboxylic acid of 3 to 6 carbon atoms, alkyl acrylate or alkyl methacrylate, and an ester containing an alkyl phenyl group, where the alkyl group has from 8 to 20 carbon atoms. This type of basic carboxyl-containing polymeric chemistry has been extended by the introduction of hydrophobic entities into the polymeric backbone. U.S. Pat. No. 4,384,096 describes an improved water soluble compound which discloses a polymer which is the reaction product of a C.sub.3 -C.sub.8 .alpha.,.beta.-ethylenically unsaturated carboxylic acid monomer, a nonionic vinyl monomer, and from 1% to 30% of a defined vinyl surfactant ester. One of the two vinyl surfactant esters shown in the patent is an alkylphenoxypoly-(oxyethylene) ethyl acrylate, wherein the alkyl group is a C.sub.8 -C.sub.16 alkyl phenyl group.
A similar variant of a polymeric thickener useful for aqueous paints is described in U.S. Pat. No. 4,421,902. This patent discloses a copolymer which can be the reaction product of monomers including methacrylic acid, ethyl acrylate, optionally a defined copolymerizable ethylenically unsaturated monomer, and a small weight percent of a polyethylenically unsaturated hydrophobic surfactant monomer. This patent further describes how a wide range of such surfactants can provide enhancement of thickening when added to an aqueous system containing the copolymer of the invention, when the emulsion copolymer is neutralized.
Pending U.S. patent application Ser. No. 08/315,000 abandoned commonly assigned to Rheox, Inc., discloses improved high performance alkali--swellable rheological additives for aqueous systems utilizing three reactive monomers one of which can include a specific hydrophobic monomer sold by Rhone-Poulenc of the type discussed herein. U.S. Pat. No. 5,336,318 describes a joint compound from which attapulgite clay can be removed, where a particular type of associative thickener compound is used in combination with traditional cellulosic thickeners including HEC. Associative thickeners used in the examples are described as the commercial products ALCOGUM L-11X and ALCO L-11. The thickener is described as functioning as a thickener and as a pseudoplasticity agent by reacting with the alkaline materials present in the joint compound, which neutralize the acidic groups present in the thickener.
The present invention was developed in response to a search for a replacement for current commercial water retention additives and the long-felt need it represented for a product which will impart the required lubricity and water retention to systems such as joint compounds without the disadvanges of cellulosics.